Blackening is a defect in a paper sheet related to the formation of discrete areas of relatively smooth surface texture during the manufacturing of newsprint. Blackened areas of interest are generated by calendering and have a darkened appearance under certain viewing conditions. Blackening is normally associated with areas of relatively high moisture content, but a direct correlation between moisture content and blackening is not likely to be achieved primarily because it is believed moisture is just one factor that contributes significantly to blackening.
No on-line equipment is available or has been devised for the measurement of blackening. In fact, it is believed that there are no detectors of any kind adapted to sense blackening. The phenomena is simply observed by the naked eye and the quality of the paper is determined by subjective judgement.
It has been proposed to examine the surface topography of a travelling paper web and to determine the topographic profile and the amount of dirt on the paper web, utilizing the technology described in U.S. Pat. No. 4,092,068 issued May 30, 1978 to Lucas and Gracovetsky. This patent discloses a sensor head wherein scattered light reflected from small illuminated areas on the surface of the travelling web are detected by a pair of detectors and added to obtain a dirt signal substantially free of topography or subtracted to obtain a topographic signal substantially free of dirt. The dirt signal provides the best indication of blackening when analysed in accordance with the teachings herein.